Heat conductive mastic



Sept. 13, 1955 c. WRIGHT ET AL HEAT CONDUCTIVE MASTIC Filed NOV. 9, 1951 will: if v .Hamww/ JNVE/VTOR.

ATTORNEY United States Patent Ofi i 2,717,839 IEAT CONDUCTIVE MASTIC Charles Wright and Warren A. Hanson, St. Paul, Minn, assignors to Seeger Refrigerator Company, St. Paul,

Inn.

Application November 9, 1951, Serial No. 255,760 4 Claims. (Cl. 106285) or attached thereto. in certain types of such freezers, metal coils are mechanically attached to the metallic liners of the cabinet and held in close contact with the sides and sometimes the bottoms thereof. Contact between the coils and the liner acts to conduct heat from the liner to the refrigerant within the coil. As the refrigerator coils are usually circular in cross section they can actually contact the surface of the liner only along a single line of contact. In order to increase the heat transfer from the liner to the coil, a beading of heat conductive mastic applied between the coil and the liner transfers heat through the mastic as well as through the actual line of contact between the coil and the liner. This mastic also serves as a heat transfer medium between the liner and the coil at points where the coil is slightly spaced from the liner, it being understood that it is diflicult to hold the coil in contact with the liner throughout its entire length.

The present invention lies in the provision of a novel type of heat conductive mastic which has been found to provide superior thermo-conductivity. As any increase in the thermo-conductivity between the coil and the liner tends to increase the over-all operating efliciency of the freezer, the mastic thus produced tends to make the freezer more eflicient than when other mastic is used.

A feature of the present invention lies in the use of a mastic between a refrigerant coil and a surface to be cooled which includes magnesium oxide combined with an oil. This combination provides better thermo-conductivity between the coil and the surface to be cooled than other mastic with which we are familiar, thereby increasing the efliciency of the freezer.

These and other objects and novel features of our in-v vention will be more clearly and fully set forth in the following specification and claims.

In the drawings forming a part of our specification:

Figure l is a sectional view through a portion of a freezer wall showing the manner in which the coil is held in close contact with the walls of the liner.

Figure 2 is an enlarged sectional view through a coil and through a portion of the liner showing the manner in which the mastic acts to connect the coil and the heat conductive liner.

The freezer A illustrated ings is only partially shown in the accompanying drawand the specific arrangement 2,717,839 Patented Sept. 13, 19 55 ice an outer casing 10 which is shown as having 11 and a bottom panel 12. 11 are ordinarily rectangularly ar-' be cylindrical in form, or any other de- The side wall panels ranged, but may sired shape.

walls 14 and the casing side walls 11. In the specific arrangement illustrated the side walls 11 of the outer casing are provided with inturned marginal flanges 19, while the side walls 14 of the liner 13 are provided with outwardly turned flanges 20 which may extend substantially flush with the flanges 19. Suitable means 21 are provided the upper edge of the refrigerator.

The cabinet is usually closed by a door be hinged to one of the side Walls of the door 23 includes an outer sheathing 24 and an inner lining panel 25. The outer covering 24 may extend marglnally toward the liner 25 and may terminate in an inwardly extending marginal flange 26 which may be from the liner 25 by a suitable gasket 27.

23 which may cabinet. The

which our mastic may be used.

The evaporator coil 29 encircles the side walls 14 of the liner 13 and lies in contact therewith throughout as 15 of the liner as illustrated.

As best shown in Figure 2 of the drawings the evaporator coil 29 usually comprises an elongated metallic tube which is held in contact with the wall 14 or 15 of the This so as good thermal contact between a relatively the coil and the liner panel. If desired the wall. generally triangular space is filled with mastic 30 to provide a large area of merely on one side 3 Viscosity,

Saybolt Degrees Name of Oil Universal Seconds 210 F. 4 Motor, heavy -27 80-155 Aircraft engine, heavy 1 23-25 100-145 Transmission Oil, heaviest 20-22' 200-220 Cylinder oils:

Light mineral 25-28 135-165 Heavy mineral 20-26 175-220 Light compoundetL 25-28 95-110 Heavy compounded 20-26 175-220 As a specific example of the mastic which we have found tobest suit the overall purposes of the present invention, we may cite the following:

Example I 58% by weight magnesium oxide I 42% by weight steam refined cylinder oil of 23-25 degrees A. P. I.

A second composition that is quite similar to Example I is presented below:

Example 11 58% by weight magnesium oxide 7 42% by weight cylinder oilof 23-25 degrees A. P. I. and

approximately 200 S; U. secs.

The proportion of magnesium oxide used in the above cited example may be increased or decreased approximately 1% without greatly affecting the. results and the percentage of oil may also be increased or decreased approximately 1%. With oil of the viscosity described above the mastic p'roducedtends to become unduly soft and to run down the walls of the liner if the proportion of oil is increased more than approximately 1%. Similarly if the proportion of oil is decreased more than approximately 1%, the mastic becomes difficult to mix and to handle with ease. Oil ofthe particular viscosity mentioned provides a better heat transfer than oil of other viscosity in the mixture. Thus while the formula of the specific example mentioned may be varied without entirely losing the-' advantage of the combination, the above formula has been found to be advantageous over other proportions.

The viscosity of the oil used may be changed and a change in the viscosity of the oil acts to somewhat change the proportion of magnesium oxide which may be usedin the combination. However, if a lighter or less viscous oil is used the mastic tends to become unduly soft at room' temperatures and an increase in the viscosity tends to make a mastic which is slightly more difficult to mix and handle and does not appear to have as good head conductive properties as the specific oil described. Similarly the oil need not be cylinder oil to produce advantageous results, although the particular oil mentioned has been found better than any other type of oil used. Thus the selection of the oil to produce best results appears important, as well as the viscosity thereof.

With reference to the particular type of oil, any of the oils set forth in the above presented list are deemed suitable, these listed oils having a gravity range of 20-28 degrees A. P. l. and a viscosity range of 530-220 S. U. secs. Actually, it is possible to employ other oils than those obtained from petroleum, such other oils ineluding coal tar derivatives and shale oils of the above viscosity range. Still further, it is sometimes of advantage to compound oils; cylinder oil, for instance, when compounded with from 57% of tallow adheres more closely to the metallic surfaces constituting the liner 13 and the coils 29. However, in general, compounding is practiced principally in order to meet viscosity specifications and therefore can be used to whatever extent is necessary to prevent the oil from running down the panel walls. A number of compounding substances are available, and it is believed that their respective characteristies are sufiiciently well known for their proper use by those familiar with the oil refining art.

Magnesium oxide appears to have better heat conductive properties in the combination mentioned than other materials of a somewhat similar character and thus is believed specifically advantageous in the combination.

While the 58 to 42 percent ratio of magnesium oxide to oil is preferred, this ratio may be varied so that the magnesium oxide covers arange of from 52-66% with the remainder being oil. When 66% magnesium oxide is used, the less viscous oils of the preceding list may be best utilized, as in the following:

Example Ill 66% by weight magnesium oxide 34% by Weight heavy motor oil of 20-27 degrees A. P. I. and a viscosity of to S. U. seconds at 210 F.

As before stated, the important thing is to produce a mastic of magnesium oxide and oil having a puttylike texture that will not permit the oil contained therein to separate out and run from the mastic at room temperatures. Of the examples presented, Example I has been found to be superior from 7 an overall characteristic standpoint; i. e. it has been discovered from actual tests that it possesses all the factors necessary to produce a mastic for the particular purpose disclosed.

In accordance with the. patent statutes, we have described the principles: of construction and operation of our heat conductive mastic, and while we have endeavored to set forth the best embodiment thereof, we desire to have it understood that obvious changes may be made within the scope of the following claims without departing from the spirit of our invention.

We claim:

1 A- heat conductive mastic consisting essentially of 52-66% by weight magnesium oxide and the remainder an oil, primarily mineral, having a viscosity range of from 80-220 Saybolt Universal seconds at 210 F.

2. A heat conductive mastic consisting essentially of substantially 58% by weight magnesium oxide and 42% by weight cylinder oil.

3-. The composition described in claim 2 in which the cylinder oil has a gravity of from 23 to 25 degrees A. P. l.

4. A- heat conductive mastic consisting essentially of substantially 58% by weight magnesium oxide and the remainder an oil selected from the class of heavy lubricating'oils' having a viscosity within the range of 80-220 Saybolt Universal seconds at 210 F.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A HEAT CONDUCTIVE MASTIC CONSISTING ESSENTIALLY OF 52-66% BY WEIGHT MAGNESIUM OXIDE AND A REMAINDER OF OIL, PRIMARILY MINERAL, HAVING A VISCOSITY RANGE OF FROM 80-220 SAYBOLT UNIVERSAL SECONDS AT 210* F. 